Automatic power control (APC) for an optical transmitter is known as a control method for maintaining constant output power of laser light from a semiconductor laser unit. In APC, a part of laser light output from a laser diode (LD) constituting a semiconductor laser unit is detected by a photodiode (PD), and a laser driving current to be supplied to the LD is controlled so that the value of a monitor current output from the PD is constant (feedback control) (see Patent Reference 1).
There also disclosed a technique of changing a voltage to be applied to a laser driving circuit in accordance with a temperature in an optical transmitter to thereby maintain constant output power of laser light from a semiconductor laser unit even upon occurrence of a temperature variation (see Patent Reference 1).